Recently, with getting larger and larger, liquid crystal displays (LCDs) are broadening their applications from personal use such as cell phones or laptops to home use such as wall mountable television sets, and in accordance with this trend, high definition, high fidelity and a wide viewing angle are required for the LCDs. In particular, since individual pixels operate independently, thin film transistor-liquid crystal displays (TFT-LCD) driven by a thin film transistor exhibit a very fast response speed of liquid crystals, and thereby make it possible to realize a high definition dynamic image and are gradually expanding boundaries of their application.
In order to be utilized as an optic switch in TFT-LCDs, liquid crystals should be initially oriented in a certain direction on a TFT layer which is disposed in the innermost part of a display cell, and a liquid crystal alignment layer is used to this end.
A current method of orienting liquid crystals in LCD, which is called as a “rubbing process,” comprises applying a thermal resistant polymer such as a polyimide on a transparent glass to form a polymer alignment layer and rubbing the alignment layer with a rapidly rotating roller wound with a rubbing cloth made of nylon or rayon to impart an orientation.
However, the rubbing process leaves mechanical scratches on a surface of a liquid crystal alignment agent or generates such a large amount of electrostatic charges that it can destruct a thin film transistor. Also, fine fibers derived from a rubbing cloth can cause a defect and are hampering an improvement in production yield.
In a bid to overcome these problems associated with the rubbing process and to make innovation in terms of productivity, a newly designed manner of orienting liquid crystals is a UV-induced (i.e., light-induced) alignment of liquid crystals (hereinafter, referred to as “photoalignment”).
Photoalignment refers to a mechanism for forming a photo-polymerizable liquid crystal alignment layer wherein pre-polarized UV rays induce a photoreaction in a photosensitivy group of a polymer, and via this process the main chains of the polymers are aligned in such a direction that liquid crystals are oriented.
Representative examples for photoalignment are the photoalignment through photopolymerization disclosed in Jpn. J. Appl. Phys., Vol. 31, 1992, 2155 by M. Schadt et al., U.S. Pat. No. 5,464,669 by Dae S. Kang et al., and Jpn. J. Appl. Phys., Vol. 34, 1995, L1000 by Yuriy Reznikov.
In the literatures and the patent as above, polycinnamate polymers such as poly(vinyl cinnamate) (PVCN) and poly(vinyl methoxycinnamate) (PVMC) were mainly used as a photoalignment polymer. In photo-aligning such a polymer, UV irradiation makes a double bond in cinnamate group go through a [2+2] cycloaddition reaction to form a cyclobutane and thereby generates anisotropy, which allows liquid crystal molecules to be aligned in one direction and leads to an orientation of liquid crystals.
With regard to conventional photoalignment polymers, Japanese Patent Laid-open Publication No. Hei11-181127 teaches a method of producing a polymer alignment layer and an alignment layer produced thereby in which main chains of polyacrylate or polymethacrylate has side chains including photoreactive groups such as cinnamate group. However, in this case, the polymer main chain has a poor thermal stability so that it has a negative impact on the stability of the alignment layer. Also, it is not easy to control a photoreaction rate via cinnamate substituent groups.
Korean Patent Laid-open Publication No. 2002-006819 discloses a method of utilizing a photoalignment layer made of polymethacrylate polymers, but the disclosed polymer has drawbacks such as a low surface hardness and a poor adhesion property.
Such a problem can be dealt with by combining a polymer having a photoreactive group with binder monomers such as acrylates or epoxy and hardening the resulting coating. In this case, an alignment layer with a higher hardness can be prepared through a light-induced reaction. However, this approach ultimately results in a lower concentration of photoreactive groups affecting the orientation of liquid crystals and thus can lead to deterioration of the orientation.